a: Field of the Invention
This invention relates to traffic bearing structures such as roads and parking lots. In particular, this invention relates to traffic bearing structures having a permeable pavement for the wear surface.
b: Description of the Related Art
Traffic bearing structures have been used by man for many years. Some examples of traffic bearing structures include roads on which we drive and parking lots where we park. There are many different types of surfaces which can be used with a traffic bearing structure, and these top surfaces are often referred to as a “wearing surface” or a “wear surface.” Some examples include asphalt, concrete, block pavers, gravel, grass, and dirt. Harder wearing surfaces can support larger loads, reduce dust and dirt, and provide a smoother surface. Examples of harder wearing surfaces include asphalt, concrete, and block pavers, all of which can be referred to as pavement. Many users prefer harder wear surfaces, and are willing to pay more for them.
Historically, harder wear surfaces tend to be water-resistant or waterproof so that rainfall simply runs off, and does not soak in to any appreciable extent. When large areas are paved over with non-permeable wear surfaces, water from rainfall tends to collect quickly and large surges of water are seen in the stormwater systems adjacent to paved surfaces. Large surges of storm water can increase the chance of flash floods, and can carry trash, solids, pollutants, and debris to local waterways. In some locations, storm water is directed to waste water treatment plants, and storm water surges can overload the waste water treatment facility. The rapid run-off of rainfall from non-permeable paved surfaces is generally considered undesirable.
Certain regulations encourage the use of permeable pavements, where permeable pavements are pavements which allow water to permeate through the pavement. Some storm water regulations can require the use of holding ponds or filtration areas for new construction projects involving nonpermeable surfaces which cover a significant portion of the land. For example, if a new shopping center were to be installed and several acres of land were to be paved to provide parking, regulations may require a holding pond to collect and hold storm water runoff and thereby reduce the peak load of stormwater exiting the new shopping center. By providing a permeable pavement on the parking area, water that strikes and collects on the parking area tends to soak through the pavement and into the ground underneath. The base then serves to hold the water and allow it to slowly percolate into the earth underneath the base. This use of permeable pavement can reduce or eliminate the need for holding ponds which are regulatory required. By reducing or eliminating the need for holding ponds, more real estate is available to be developed. This can allow for more retail space and more parking, and provides a higher end use for the real estate. This allows more productive use of the land while still reducing the peak flows from stormwater.
A permeable pavement used in a traffic bearing structure, such as a parking lot, has to be able to handle the traffic load without being damaged. The permeable pavement has to be strong enough to support the cars, trucks and other vehicles traveling on the parking lot. Much of the strength of a permeable pavement comes from the base underneath it, and so a stronger, more stable base can increase the traffic load a permeable pavement can support. Roads tend to have heavier vehicles and higher traffic flow than parking lots, and often require higher load ratings. Providing a base which is strong enough to support the permeable pavement and yet capable of allowing water to flow through it can facilitate the use of permeable pavements in services which require higher load ratings. In many cases, permeable pavements have been used for driveways, certain parking lots, and other uses with limited load ratings.
The base underneath a permeable pavement is often an aggregate. Certain types of aggregate are known and available. One such aggregate is referred to as a dense graded base. Another type of aggregate available is an open graded base. The dense graded base tends to be very stable, but has relatively low permeability, whereas the open graded base tends to have higher permeability but less stability. The base used beneath many permeable pavements includes at least two different types of aggregate, where each type is independently compacted. Different types of aggregate can be differentiated by the size distribution of the particles within each type of aggregate. Providing a base which is permeable, strong, and stable for use with permeable pavements is desirable.